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How do I connect a crystal's ground island to ground plane?

kukaahi

Following a suggestion, I created a ground island around my clock using a solid pour polygon.  The polygon is supposed to be isolated at all points save for a pin on the micro that is both connected to the island ground and the ground plane.  In this example the island is named XT2GND.  Attached are snippets of the polygon in layout, and the schematics of the crystal plus where the crystal connects to the micro from files downloaded from TI.  As you can see in the board layout the highlighted trace is the XT2GND connection to the micro and opposite the trace connecting to the same pin is a trace tying the pin to GND.  The related schematic represents the island being connected to ground via text while the portion of the micro's schematic shows the pin is tied to ground.

 

       

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  • 0

    On 02/10/13 18:58, Matthew Chapman wrote:

    Following a suggestion, I created a ground island around my clock using

    a solid pour polygon.  The polygon is supposed to be isolated at all

    points save for a pin on the micro that is both connected to the island

    ground and the ground plane.  In this example the island is named

    XT2GND.  Attached are snippets of the polygon in layout, and the

    schematics of the crystal plus where the crystal connects to the micro

    from files downloaded from TI.  As you can see in the board layout the

    highlighted trace is the XT2GND connection to the micro and opposite the

    trace connecting to the same pin is a trace tying the pin to GND.  The

    related schematic represents the island being connected to ground via

    text while the portion of the micro's schematic shows the pin is tied to

    ground.

     

    I'm sorry, but I really can't figure out what you're asking here.

     

    If you want the net you've called "XT2GND" to be ground then you need to

    connect it on the schematic, with a green line, not just put some text

    there. If you really need it to be a differently named net then use a

    "short" component, but I don't think you do need that.

     

    If you want the island to be only connected in one place, you can do so

    by removing the three vias and the spurious trace. The capacitors are on

    the bottom so they won't connect to the polygon (which is on the top)

    and you can instead route them to the via that's under the micro.

     

    That said, I'm not convinced there's any harm in the ground island

    around the clock being connected to ground in more than one place.

     

  • 0

    Hi Matthew,

     

    Another point to check is the reason for creating a ground plane underneath the crystal.

    Unless there is an explicit reason for this (i.e. the documentation with the IC that you are using, which contains the oscillator circuit that the crystal is part of, tells you to do so, or it is in the evaluation PCB gerber plot) then you very likely shouldn't do this.